Multi-level phase-shift-keying (PSK) offers high spectral efficiency transmission in coherent optical communication systems. Quaternary PSK (QPSK) format, in particular, has recently received much attention. An optical QPSK signal can be generated, e.g., by an integrated LiNbO3 quadrature modulator (QM) with two parallel Mach-Zehnder modulators (MZMs) nested in a MZ interferometer. Each MZM is driven to produce a binary PSK (BPSK) signal. An optical QPSK signal is produced when the two MZMs are biased at their null transmission points and the MZ interferometer is biased at the quadrature phase (π/2). FIG. 1 shows a schematic of a quadrature modulator 1 known in the prior art. The principle of its operation is as follows. Input optical beam 2 is splitted into two arms of the MZ interferometer by a splitter 3. Two Mach-Zehnder modulators 4 and 5 are placed in parallel; each MZM being located in each arm of the MZ interferometer. The biases of the MZMs are controlled by control signals 7 and 8 and driven by RF data signals 8 and 9. The Phase port of the QM 10 controls relative phase shift between the arms of the MZ interferometer.
In modern communication systems operating at a speed exceeding 10 Gbits/s, a precise stabilization of QPSK modulators is required. There is a need for an automatic feedback control loop that searches for these biases and phase operating points of the QM at initial startup and maintains them during operation.